The present invention relates to a method of electronic brake force distribution (EBD) on a two-axle, four-wheel vehicle during cornering, wherein a respective individual wheel reference speed derived from at least one wheel sensor signal is compared with a vehicle reference speed to determine the activation of electronic brake force distribution on a rear wheel.
Methods of this type are known in manifold ways and used in first line to prevent the rear wheels from locking prior to the front wheels during braking operations. When needed, a pressure maintenance phase or a pressure reduction phase is initiated in pressure-fluid operated brake systems on the rear-axle brakes, which avoids that the brake slip of the rear-axle brakes continues to increase and prevents locking of the rear wheels. A customary manner of detecting whether there is a need for initiating electronic brake force distribution includes comparing the wheel speeds of the rear wheels (wheel speed always means the wheel circumferential speed) with a vehicle reference speed that is determined from all wheel speeds. When an individual wheel speed of a rear wheel during braking is considerably slower than the vehicle reference speed, a great amount of brake slip is identified, and a pressure maintenance phase is introduced, for example, in a pressure-fluid operated brake system so that the brake pressure on any one or on both of the rear wheels is not further increased.
To determine a vehicle reference speed, it is customary to take into account the individual wheel speed of the slowest wheel during acceleration of the vehicle. During braking or deceleration, the individual wheel speed of the respectively fastest wheel is adopted as vehicle reference speed. Also, there are considerations to take into account the second slowest wheel or the second fastest wheel to determine the vehicle reference speed. However, the principle always resides in that the wheel representative of the vehicle reference speed has the lowest amount of slip, if possible. When cornering, the problem is that of course the outward wheels in a curve have a higher individual wheel speed than the curve-inward wheels. Accordingly, the individual wheel speed of one of the curve-outward wheels is taken as the vehicle reference speed. The brake slip calculated by the electronic controller, which is computed of the difference between the vehicle reference speed and the individual wheel speed of the rear wheels is, thus, faulty due to the influence of geometry during cornering. The effect is that a pressure maintenance phase is introduced too early during electronic brake force distribution and, under certain circumstances, an excessive amount of pressure is discharged during control.
However, if the threshold of entry into a pressure maintenance phase was increased, the curve-outward rear wheel would possibly be overbraked, which might cause loss in cornering forces.
An object of the present invention is to provide a method of the type mentioned hereinabove for electronic brake force distribution which compensates the influence of geometry during cornering.
This object is achieved by assigning an individual wheel reference speed instead of an individual wheel speed to the curve-inward rear wheel when an information about cornering is obtained. The individual wheel reference speed is calculated as a sum of the individual wheel speed and a compensation speed.